We have a webserver (Debian Stretch) that hosts multiple domains. Unfortunately, our developers tend to use debugging (adding code that writes something in a log file) and forget about it, even after it no longer serves a purpose.

Of course, the free disk space continuously shrinks over time but as the logs still are not that big, a simple "du" won't be able to find it. Also, a cronjob that mails a daily report of directory sizes isn't useful if you don't know what directory to look at.